1. Field of the Invention
This invention relates to a method for curing an organoalkoxysilane compound, and more particularly, to a method for converting an organoalkoxysilane compound to organopolysiloxane characterized by using a selected phosphite as a catalyst.
2. Description of the Prior Art
An organoalkoxysilane compound which, upon heating, forms a siloxane bond to provide a cured product having a network structure finds utility in many applications such as an electrical insulating material, electrical insulating varnish and mar resistant paint. The compound is useful as a base material that has desired properties such as high heat resistance and water resistance as well as good weather-ability and electrical insulation. In recent years, the organoalkoxysilane compound if drawing researchers' attention as a material that forms a transparent mar resistant coating on a transparent plastic shaped article, for instance, a shaped article of polycarbonate, polymethyl methacrylate, cellulose butyrate, polyvinyl chloride or polystyrene. Because of their transparency and lightweightness, these plastic shaped articles are expected to replace glass in many applications, but the fact is that, to date, their use has been limited because of their vulnerability to abrasion and loss of transparency.
Several methods have been proposed to eliminate such defect of the transparent plastic shaped article by coating its surface with a transparent paint to provide a protective film. Silicone paints are most popular as the paint that forms a transparent protective coating. One example of such methods is disclosed in U.S. Pat. No. 3961977 wherein a plastic shaped article is coated with a paint comprising a partial hydrolyzate of epoxyalkylalkoxysilane and aminoalkylalkoxysilane plus an organic solvent. In U.S. Pat. No. 4,167,537 a paint is proposed comprising said reaction product of epoxyalkylalkoxysilane and aminoalkylalkoxysilane plus a melamine/alkyd resin, and optionally a derivative of acrylic acid. However, these paints exhibit high mar resistance only after they are cured under very severe conditions, such as at 130.degree. C. for a period of 4 hours. Such severe conditions of thermal curing not only limit the kind of the substrate shaped article but sometimes they also cause the discoloration (yellowing) problem due to thermal deterioration of the coating.
As a further disadvantage, an organoalkoxysilane compound in storage gradually absorbs atmospheric moisture and this deactivates (i.e. renders inactive) the active group (generally an alkoxysilane group) contributory to the curing reaction, and this may result in insufficient heat curing. If the organoalkoxysilane compound is used as a paint, such disadvantage is the cause of its characteristic deterioration, namely, considerable decrease in the mar resistance of the final coating it forms. To inhibit such deterioration, the prior art technique has incorporated in the compound a catalyst consisting of a mineral acid such as phosphoric acid or sulfuric acid or a hydroxide of alkali metal. However, these catalysts exhibit their effect even during storage of the paint and as a result, the storage stability of the paint is decreased to such an extent that it is no longer commercially acceptable.